Cold-rolled steel sheet having excellent strain aging hardening properties and method for producing the same

ABSTRACT

The present invention provides a cold-rolled sheet exhibiting superior strain-aging hardenability which is suitable for use in automobile bodies, and a method for manufacturing the same. In particular, a slab including, by mass, not more than 0.15% carbon, not more than 0.02% Al, and 0.0050% to 0.0250% nitrogen in which Si+Mn/5+10P is adjusted to less than 0.44 and the ratio N/Al is adjusted to not less than 0.3 is hot-rolled at a finish-rolling delivery temperature FDT of not less than 800° C., and then coiled at a temperature of not more than 650° C. Next, after cold-rolling, continuous-annealing at a temperature between the recrystallization temperature and 950° C., primary cooling for cooling to a temperature zone of not more than 500° C., and overaging in the temperature zone of 350° C. to 450° C. for a retention time of not more than 30 seconds are performed to prepare a steel sheet containing not less than 0.0010% of solute nitrogen and having a structure in which the ferrite phase with the grain size of not more than 15 μm is included at a ratio of not less than 90%, the remainder of the structure being the pearlite phase. The resulting steel sheet has superior strain-aging hardenability in which the tensile strength is less than 440 MPa and the yield ratio is less than 70%.

TECHNICAL FIELD

[0001] The present invention relates to cold-rolled steel sheetsprimarily suitable for use in automobile bodies. In particular, thepresent invention relates to a cold-rolled steel sheet exhibiting atensile strength (TS) of less than 440 MPa and having superiorstrain-aging hardenability and to a method for manufacturing the same.While various grades of steel sheets from those suitable for lightprocessing to those suitable for extensive deep drawing are availablefor the steel sheet for automobile bodies, the cold-rolled steel sheetof the present invention is suitable for use in processes which requirerelatively low-grade steel sheets with suitable processability. Thecold-rolled steel sheets of the present invention are suitable for awide range of uses from use in relatively light processes such asforming pipes by light bending or roll forming to use in relativelysevere drawing. In this invention, the term “steel sheets” includessteel strips.

[0002] In the present invention, the statement “superior strain-aginghardenability” means that the deformation stress increment before andafter an aging treatment is not less than 80 MPa and that the tensilestrength increment before and after the strain aging treatment(predeformation+aging treatment) is not less than 40 MPa, the agingtreatment being performed at a temperature of 170° C. held for 20minutes and being conducted after predeformation at a tensile strain of5%. The deformation stress increment, which is the difference betweenthe yield strength before the aging treatment and after the agingtreatment, is also referred to as a BH increment. The tensile strengthincrement, which is the difference between the tensile strength beforethe predeformation and after the aging treatment, is hereinafterrepresented by ΔTS.

BACKGROUND ART

[0003] With current gas-emission regulations concerning globalenvironmental problems, reducing body weight of automobiles has become amatter of vital importance. In order to reduce the body weight ofautomobiles, increasing the strength of the steel sheets used at a largeamount, i.e., applying high-strength steel sheets, and reducing thethickness of the steel sheets are effective.

[0004] Unfortunately, steel sheets exhibiting significantly highstrength suffer from the following problems during press forming inmanufacture of the automobile components:

[0005] 1. degradation of shape fixability

[0006] 2. occurrence of cracking and necking during forming due todegraded ductility.

[0007] In order to overcome the above problems, steel sheets made fromultra-low carbon steels in which the amount of carbon in thesolid-solution state remaining in the final product is controlled withina suitable range is known among cold-rolled steel sheets for use inexterior panels. This type of steel sheet is kept soft during pressforming so as to secure shape fixability and ductility. Dent resistanceis achieved by increasing the yield stress resulting from a strain-aginghardening phenomenon which occurs during a paint-baking process in whicha temperature of 170° C. is held for about 20 minutes. This type ofsteel sheet having carbon in solid-solution in the steel is soft duringpress forming. In a paint-baking process subsequent to the press formingprocess, dislocations caused by press forming are fixed by solutecarbon, thereby increasing the yield stress.

[0008] In this type of steel sheet, however, the increase in the yieldstress caused by strain-aging hardening is kept low in order to preventstretcher strain during press forming which will cause surface defects.Thus, actual contribution of this steel sheet to weight reduction of thecomponent is small.

[0009] In order to reduce the weight of the components, both theincrease in the yield stress due to strain aging and the increase in thestrength characteristics after progressed deformation are necessary. Inother words, an increase in the tensile strength after strain aging isrequired.

[0010] In contrast, for uses where good appearance is not required, asteel sheet in which solute nitrogen is used to improve the bakehardening increment, and a steel sheet in which the bake hardenabilityis further improved by the composite structure comprising ferrite andmartensite have been suggested.

[0011] For example, Japanese Unexamined Patent Application PublicationNo. 60-52528 discloses a hot-rolling process in which a steel containingC: 0.02% to 0.15%, Mn: 0.8% to 3.5%, P: 0.02% to 0.15%, Al: not morethan 0.10%, and N: 0.005% to 0.025% is coiled at a temperature of notmore than 550° C. and a method for manufacturing a high-strength steelsheet exhibiting good ductility and spot weldability in whichcontrolled-cooling annealing is performed after cold rolling. The steelsheets manufactured by the technology described in Japanese UnexaminedPatent Application Publication No. 60-52528 has a mixed structurecomprising a phase of low-temperature transformation products mainlyincluding ferrite and martensite and exhibits good ductility. In thissteel sheet, strain aging during paint baking caused by deliberatelyadded nitrogen is utilized to obtain high strength.

[0012] In the technology described in Japanese Unexamined PatentApplication Publication No. 60-52528, the increase in yield strength YSdue to strain-aging hardening is large but the increase in the tensilestrength TS is small. Moreover, a variation in mechanicalcharacteristics such as a variation in the increase of the yield stressYS is significantly large. Thus, the thickness of the steel sheetscannot be reduced by as much as currently required to reduce the weightof the automobile components.

[0013] Japanese Patent Publication No. 5-24979 discloses abake-hardenable high-tension cold-rolled thin steel sheet having acomposition of C: 0.08% to 0.20%, Mn: 1.5% to 3.5% with the balancebeing Fe and unavoidable impurities. The structure of this steel sheetcomprises homogenous bainite containing 5% or less of ferrite, orbainite partially including martensite. The cold-rolled steel sheetdisclosed in Japanese Patent Publication No. 5-24979 has a structureprimarily including bainite obtained by quenching in the temperaturerange of 400° C. to 200° C. during cooling step after continuousannealing, the quenching being followed by slow-cooling. The steel sheetattains, through this structure, a high bake-hardening increment whichhas not been attained before.

[0014] In the steel sheet described in above Japanese Patent PublicationNo. 5-24979, although the yield strength increases after paint-bakingand a high degree of bake hardening which has never been achieved beforeis obtained thereby, the tensile strength is not increased. When appliedto components requiring strength, improvements in fatigue resistance andcrash resistance after forming cannot be expected. Thus, there is aproblem in that the steel sheet cannot be used where high fatigueresistance and crash resistance are required.

[0015] Moreover, the conventional steel sheets described above, thoughexcellent in strength when evaluated by simple tensile testing afterpaint-baking treatment, exhibit a significantly large variation instrength after the steel sheets have been subjected to plasticdeformation under actual press conditions. Thus, these steel sheets arenot necessarily suitable for the components requiring high reliability.

[0016] The present invention aims to overcome the limitations of theabove-described related art and to provide a cold-rolled steel sheetexhibiting good formability, stable quality, and superior strain-aginghardenability, the steel sheet having sufficient strength for use inautomobile components after the steel sheet is formed into automobilecomponents, thus helping to reduce the weight of automobile bodies. Amethod for commercially manufacturing the steel sheets at low cost isalso provided. The goal of the present invention is to achieve astrain-aging hardenability satisfying an BH increment of 80 MPa or moreand ΔTS of 40 MPa or more with the aging conditions being a temperatureof 170° C. held for 20 minutes after predeformation at a tensile strainof 5%.

DISCLOSURE OF INVENTION

[0017] To achieve these objects, the present inventors have manufacturedvarious steel sheets while varying composition and manufacturingconditions, and examined and evaluated many of their characteristics. Asa result, the inventors have found that by using nitrogen, which has notbeen advantageously used in the field where high processability isrequired, as a strengthening chemical element, and by effectively makinguse of a large strain-aging hardening phenomenon caused by thisreinforcing chemical element, both improvement in formability andincrease in the strength after forming can be easily achieved.

[0018] Furthermore, the inventors have found that, in order to fullyutilize the strain-aging hardening phenomenon caused by nitrogen, thestrain-aging hardening phenomenon should be effectively linked to thepaint-baking conditions for automobiles and further to the conditions ofheat treatment after forming, and that the hot-rolling conditions,cold-rolling conditions, and cold annealing conditions must be optimizedto control the microstructure of the steel sheet and the amount ofsolute nitrogen to within a predetermined range. The inventors have alsofound that control of the aluminum (Al) content relative to the nitrogen(N) content in the composition is essential for stable strain-aginghardening using nitrogen. Moreover, the inventors have found that whenthe microstructure of the steel sheet includes ferrite as the primaryphase and has an average grain diameter of not more than 15 μm, theproblem of room-temperature aging encountered in the known art can beovercome, and nitrogen can be fully utilized.

[0019] In other words, the inventors have found that if nitrogen is usedas the strengthening element, the Al content is controlled in a suitablerange corresponding to the N content, and the hot-rolling conditions,the cold-rolling conditions, and the cold annealing conditions areoptimized to adjust the microstructure and solute nitrogen, then theresulting steel sheet will exhibit a far superior formability comparedto known solid-solution strengthening type C-Mn based steel sheet andprecipitation strengthening type steel sheets and will obtain superiorstrain-aging hardenability which has been absent in these known steelsheets.

[0020] The steel sheet of the present invention after paint-bakingprocess exhibits a higher strength as determined by a simple tensiletest compared to known steel sheets. The variation in the strength afterplastic deformation under the actual pressing conditions is small, andreliable component strength characteristics can be achieved. Forexample, in the region where a large strain is applied, the thickness issmall but degree of hardening is large compared to other regions of thesteel sheet, resulting in uniformity evaluated in terms of surcharge,i.e., the sheet thickness multiplied by the strength. Thus, the strengthof the component can be stabilized.

[0021] Based on the above findings, further examinations have beenconducted to complete the present invention.

[0022] A first invention provides a cold-rolled steel sheet havingsuperior strain-aging hardenability and exhibiting a tensile strength ofless than 440 MPa and a yield ratio YR of less than 70%, the steel sheetcomprising, by mass: not more than 0.15% carbon; not more than 0.4%silicon; not more than 2.0% manganese; not more than 0.04% phosphorous;not more than 0.02% sulfur; not more than 0.02% aluminum; and 0.0050% to0.025% nitrogen, wherein the Si content, the Mn content, and the Pcontent satisfy relationship (1):

Si+Mn/5+10P<0.44  (1)

[0023] where Si, Mn, P represent contents, in terms of percent by mass,of corresponding elements. The ratio N/Al is not less than 0.3, thecontent of nitrogen in the state of solid solution is not less than0.0010%, the balance of the composition is Fe and unavoidableimpurities. The steel sheet has a structure comprising the ferrite phaseand the pearlite phase, the area ratio occupied by the ferrite phasebeing not less than 90%, the average grain size of the ferrite phasebeing not more than 15 μm. Preferably, the cold-rolled steel sheet has athickness of not more than 3.2 mm. Preferably, the first inventionfurther includes at least one group selected from Group a to and Group dbelow:

[0024] Group a: at least one of Cu, Ni, Mo, and Cr, the total contentbeing not more than 1.0%;

[0025] Group b: at least one of Nb, Ti, and V, the total content beingnot more than 0.1%; and

[0026] Group c: at least one of Ca and REMs, the total content being0.0010% to 0.010%.

[0027] A second invention provides a method for manufacturing acold-rolled steel sheet having superior strain-aging hardenability andexhibiting a tensile strength of less than 440 MPa and a yield ratio YRof less than 70%, the method comprising:

[0028] a hot-rolling process comprising: heating a steel slab to atemperature of not less than 1,000° C., the steel slab comprising, bymass: not more than 0.15% carbon; not more than 0.4% silicon; not morethan 2.0% manganese; not more than 0.04% phosphorous; not more than0.02% sulfur; not more than 0.02% aluminum; and 0.0050% to 0.025%nitrogen, wherein the Si content, the Mn content, and the P contentsatisfy relationship (1):

Si+Mn/5+10P<0.44  (1)

[0029] where Si, Mn, P represent contents, in terms of percent by mass,of corresponding elements, the ratio N/Al being not less than 0.3;rough-rolling the heated slab into a sheet bar; finish-rolling the sheetbar at a finish-rolling delivery temperature of not less than 800° C.;and coiling the resulting sheet bar at a temperature of not more than650° C. into a coiled hot-rolled sheet,

[0030] a cold-rolling process comprising pickling and cold-rolling theresulting hot-rolled sheet to prepare a cold-rolled sheet, and

[0031] a cold-rolled sheet annealing process comprising: annealing theresulting cold-rolled sheet at a temperature between therecrystallization temperature and 950° C. for a holding time of 10 to120 seconds; cooling at a cooling rate of 10 to 300° C./s to atemperature zone of not more than 500° C.; and optionally overaging inthe temperature zone of 350° C. to 500° C. for a retention time of notless than 20 seconds. These processes are sequentially performed. In thesecond invention, quenching at a cooling rate of not less than 30° C./sis preferably performed before the coiling and after the finish rolling.

[0032] In the second invention, more preferably, temper rolling orleveler processing is performed at an elongation of 1.0% to 15%subsequent to the cold-rolled sheet annealing process.

[0033] Moreover, a sheet bar is preferably joined to sheet barspreceding and following that sheet bar between the rough rolling and thefinish rolling. In the second invention, either one or both of a sheetbar edge heater for heating the ends the sheet bar in the widthdirection and a sheet bar heater for heating the ends and entire lengthof the sheet bar in the longitudinal direction are preferably usedbetween the rough rolling and the finish rolling.

BEST MODE FOR CARRYING OUT THE INVENTION

[0034] First, the composition restrictions of the steel sheets of thepresent invention is explained. Hereinafter, “mass %” is simplyrepresented as “%”.

[0035] C: Not More Than 0.15%

[0036] Carbon (C) increases the strength of the steel sheets. In orderfor the average grain size of ferrite, which is the essentialconstituent of the present invention, to be not more than 15 μm and inorder to achieve the desired strength, the carbon content is preferablynot less than 0.005%. At a carbon content exceeding 0.15%, theproportion of carbides in the steel sheets becomes excessive,formability is degraded due to a degraded ductility, and spotweldability and arc weldability are significantly impaired. From thepoint of view of formability and weldability, carbon is limited to notmore than 0.15%. From the point of view of press formability, the carboncontent is preferably not more than 0.08%. For use which requires highductility, the carbon content is preferably not more than 0.05%.

[0037] Si: Not More Than 0.4%

[0038] Silicon (Si) is a useful element capable of increasing thestrength of the steel sheets without significantly decreasing theductility of the steel. The Si content is preferably not less than0.005% in order to obtain such a useful effect. The Si content issuitably adjusted according to the desired strength. On the other hand,silicon significantly raises the transformation point during hotrolling, thereby making it difficult to secure the desired quality andshapes, and adversely affects the appearance of the steel sheet surface,i.e., the surface characteristics, chemical conversion treatability, andthe like. In the present invention, the Si content is limited to notmore than 0.4%. At a Si content of not more than 0.4%, significantelevation of the transformation point can be inhibited by adjusting theamount of additive manganese, and satisfactory surface characteristicscan be securely obtained. For uses where good appearance is specificallyrequired, the Si content is preferably not more than 0.2%.

[0039] Mn: Not More Than 2.0%

[0040] Manganese (Mn) is effective for preventing hot brittleness and ispreferably added corresponding to the sulfur content. Manganese greatlycontributes to the making of fine grains which is the essentialrequirement of the present invention, and is preferably deliberatelyadded to improve the material characteristics. From the point of view ofstably fixing sulfur, the Mn content should be not less than 0.2%. Also,manganese increases the strength of the steel sheets. In the case wherea relatively high strength is required, the Mn content is preferably notless than 1.2%, and, more preferably, not less than 1.5%. With the Mncontent being increased to such levels, the variation in thestrain-aging hardenability and in the mechanical characteristics of thesteel sheets corresponding to the variation of the manufacturingconditions such as hot rolling conditions can be minimized, therebyeffectively stabilizing the quality.

[0041] Moreover, manganese brings down the transformation point duringhot rolling. When used in combination with silicon, the elevation of thetransformation point caused by silicon can be counteracted. Especiallyin the products with a small sheet thickness, the quality and the shapesare readily changed by the change in the transformation point.Accordingly, the manganese content and the silicon content need to beexactly balanced. In view of the above, M/Si is preferably not less than3.0.

[0042] At a high Mn content exceeding 2.0%, the deformation resistanceof the steel sheet at elevated temperatures tends to increase, and thespot weldability and formability of the welded parts tend to degrade.Moreover, since formation of ferrite is inhibited, there tends to be alarge decrease in ductility. Accordingly, the Mn content is limited tonot more than 2.0%. For uses where high corrosion resistance andformability are required, the Mn content is preferably not more than1.7%.

[0043] P: Not More Than 0.04%

[0044] Phosphorous (P) contributes to solid-solution hardening of thesteel. The P content is preferably not more than 0.001% to achieve suchan effect and is suitably adjusted according to the desired strength.The P content is preferably 0.015% to yield a large increase in thestrength by solid-solution hardening. At an excessive P content, thesteel becomes brittle and the stretch-flanging processability of thesteel sheet is degraded thereby. Since the segregation of phosphoroustends to occur in the steel sheet, the welded parts become brittle.Accordingly, the P content is limited to not more than 0.04%. In thecase where the stretch-flanging processability and the toughness of thewelded parts are especially important, the P content is preferably notmore than 0.02%.

[0045] Si, Mn, P: the Range Satisfying Relationship (1):

Si+Mn/5+10P<0.44  (1)

[0046] where Si, Mn, and P represent the contents, in terms of percentby mass, of the corresponding elements.

[0047] All of silicon, manganese, and phosphorous increase the strengthby solid-solution hardening. Since the structure of the presentinvention is limited to the structure comprising the ferrite phase andthe pearlite phase and the tensile strength is limited to less than 440MPa, the contents of Si, Mn, and P are limited to the range whichsatisfies both the above described limitations and relationship (1).With the left side of relationship (1), i.e., A=Si+Mn/5+10P, being 0.44or more, the strength is excessively increased, and the desiredductility cannot be obtained. Moreover, degradation in the weldabilityof the steel and in the appearance of the steel sheet surface willoccur.

[0048] Although the detailed mechanism is unknown, the aginghardenability is degraded when the value A is 0.44 or more. In order tosecure superior strain-aging hardenability, the value A should be lessthan 0.44.

[0049] S: Not More Than 0.02%

[0050] Sulfur (S) is present in the steel sheet as an inclusion andcauses degradation of ductility and corrosion resistance of the steelsheet. In the present invention, the S content is limited to not morethan 0.02%. For use which requires particularly high workablity, the Scontent is preferably not more than 0.015%. In the case where high levelof stretch-flanging processability is required, the S content ispreferably not more than 0.008%. In order to suitably maintain thestrain-aging hardenability at a high level, the S content is preferablydecreased to not more than 0.008%, although the detailed mechanism isunknown.

[0051] Al: Not More Than 0.02%

[0052] Aluminum (Al) effectively functions as a deoxidizing agent andimproves the cleanliness of the steel. Aluminum also contributes to afine structure in the steel sheet. In the present invention, the Alcontent is preferably not less than 0.001%. On the other hand, anexcessive Al content will degrade the steel sheet surfacecharacteristics. Moreover, the solute nitrogen which is the essentialconstituent of the present invention is decreased, causing a shortage ofsolute nitrogen which contributes to the strain-aging hardeningphenomenon and variation in the strain-aging hardenability which is thefeature of the present invention. In view of the above, in the presentinvention, the Al content is limited to not more than 0.02%, which islow. From the point of view of stabilizing material characteristics, theAl content is preferably not more than 0.015%.

[0053] N: 0.0050% to 0.025%

[0054] Nitrogen (N) increases the strength of the steel sheet bysolid-solution hardening and strain-aging hardening, and is the mostimportant chemical element in the present invention. Nitrogen alsobrings down the transformation point of the steel and can be suitablyused for rolling of thin plates which is incompatible with a temperaturesignificantly smaller than the transformation point, therebycontributing to stable operations. In the present invention, the Ncontent and the manufacturing conditions are suitably controlled tosecure a sufficient amount of solute nitrogen in the cold-rolledproducts or plating products. This ensures a sufficient increase in thestrength (YS and TS) caused by the solid-solution strengthening, andthus strain-aging hardening and the mechanical characteristicrequirements of the steel sheet of the present invention, i.e., thebake-hardening increment (BH increment) of not less than 80 MPa and thetensile strength increment (ΔTS) of not less than 40 MPa, can besecurely satisfied.

[0055] At a N content of less than 0.0050%, the above-described effectof strength increase is not stably achieved. At a N content exceeding0.025%, the internal defect rate and the surface defect rate of thesteel sheet become high. Moreover, fractures frequently occur in theslab during continuous casting. In view of the above, the N content isset in the range of 0.0050% to 0.025%. From the point of view ofimproving the yield and the stability of the material in the wholemanufacture process, the N content is preferably in the range of 0.0070%to 0.020%. The nitrogen will not adversely affect the weldability suchas the spot weldability and the arc weldability, if limited within thecontent range of the present invention.

[0056] N in the Solid-Solution State: Not Less Than 0.0010%

[0057] In order for the cold-rolled products to securely exhibitsufficient strength and to attain sufficient strain-aging hardening bynitrogen (N), the content (concentration) of the nitrogen in thesolid-solution state (also referred to as “solute nitrogen”) in thesteel sheet must be not less than 0.0010%.

[0058] The amount of solute nitrogen is determined by subtracting theamount of precipitated nitrogen from the total amount of nitrogen in thesteel. For the method of analyzing the amount of precipitated nitrogen,an electroextraction analysis employing chronoamperometry is effectiveaccording to the results of the comparative examination of variousanalytical methods conducted by the inventors. For the method ofdissolving the base steel employed in the extraction analysis, an aciddigestion method, a halogen method, and an electrolytic decompositionmethod are available. The electrolytic method can stably dissolve thebase steel without decomposing highly unstable microscope precipitationmatters such as carbides and nitrides. An acetylacetone-basedelectrolyte is used to perform electrolysis at a constant potential. Inthe present invention, the amount of precipitated nitrogen measured bythe chronoamperometry exhibited the best correspondence with the actualcomponent strength.

[0059] In view of the above, in the preset invention, the amount ofnitrogen in the residue is measured by chemically analyzing the residueextracted by the chronoamperometry and is defined as the amount ofprecipitated nitrogen.

[0060] In order to attain higher BH increments and higher ΔTS, theamount of solute nitrogen is preferably not less than 0.0020% and, foreven higher BH increments and ΔTS, not less than 0.0030%.

[0061] N/Al (the Ratio of the N Content to the Al Content): Not LessThan 0.3

[0062] In order for the solute nitrogen to stably remain in the productat an amount of not less than 0.0010%, restricting the amount ofaluminum which firmly fixes nitrogen is necessary. Examination of thesteel sheets in which the combination of the N content and Al contentare widely varied within the composition ranges of the present inventionshows that, in order for the content of the solute nitrogen to be notless than 0.0010% in the cold-rolled products and plating products, N/Alneeds to be not less than 0.3 if the Al content is kept as low as notmore than 0.02%. In other words, the Al content is restricted to notmore than (the N content)/0.3.

[0063] In the present invention, in addition to the above-describedcomponents, at least one group selected among Group a to Group c belowis preferably contained:

[0064] Group a: at least one of Cu, Ni, Cr, and Mo, the total contentbeing not more than 1.0%;

[0065] Group b: at least one of Nb, Ti, and V, the total content beingnot more than 0.1%; and

[0066] Group c: at least one of Ca and REMs, the total content being inthe range of 0.0010% to 0.010%.

[0067] Elements of Group a: Copper (Cu), nickel (Ni), chromium (Cr) andmolybdenum (Mo) contribute to increasing the strength withoutsignificantly decreasing the ductility of the steel sheet. Such aneffect can be obtained at a Ni content of not less than 0.01%, a Crcontent of not less than 0.01%, and a Mo content of not less than 0.01%.They may be used alone or in combination as required. At excessiveamounts, the deformation resistance at elevated temperatures increases,thereby degrading the chemical conversion treatability and the surfacetreatability in a broader sense. Moreover, the welded parts arehardened, thereby degrading the formability thereof. In view of theabove, the total content of the element(s) in Group a is preferably notmore than 1.0%.

[0068] Elements in Group b: Niobium (Nb), titanium (Ti), and vanadium(V) contribute to making fine and homogeneous grains. Such an effect canbe attained at a Nb content of not less than 0.002%, a Ti content of notless than 0.002%, and a V content of not less than 0.002%. They may beused alone or in combination as required. At excessive amounts, thedeformation resistance at elevated temperatures increases and thechemical conversion treatability and the surface treatability in abroader sense are degraded. In view of the above, the total content ofthe element(s) in Group b is preferably not more than 0.1%.

[0069] Elements in Group c: Calcium (Ca) and rare earth metal elements(REMs) help to control the forms of the inclusions. Especially in thecase requiring high stretch-flanging formability, it is preferred thatthese elements be used alone or in combination as required. At a totalcontent of the element(s) in Group c being less than 0.0010%, the effectof controlling forms of the inclusions is insufficient. On the otherhand, when the total content exceeds 0.010%, surface defects becomenoticeable. In view of the above, the total content of the element(s) inGroup c is preferably limited to the range of 0.0010% to 0.010%.

[0070] The structure of the steel sheet of the present invention willnow be described.

[0071] Area Ratio of the Ferrite Phase: Not Less Than 90%

[0072] It is intended that the cold-rolled sheet of the presentinvention be used as the steel sheet for automobiles requiring highprocessability. In order to secure the required ductility, the structurethereof includes the ferrite phase at an area ratio of not less than90%. If the area ratio of the ferrite phase is less than 90%, theductility required as the steel sheet for automobiles having highprocessability is rarely obtained. Moreover, although the details of themechanism are unknown, high strain-aging hardenability cannot be stablyachieved at an area ratio of the ferrite phase of less than 90%. Theother phase not of the ferrite phase is the pearlite phase.

[0073] Average Grain Size of the Ferrite Phase: Not More Than 15 μm

[0074] In the present invention, the larger grain size of the grain sizecalculated from the cross-section image of the structure by astereometry regulated by ASTM and the nominal grain size obtained fromthe cross-section image of the structure by the sectioning methodregulated by ASTM (for example, see Umemoto et al.: NETSUSHORI, vol. 24(1984), 334) is selected as the crystal diameter of the presentinvention.

[0075] The cold-rolled steel sheet of the present invention as a productincludes a predetermined amount of solute nitrogen. The investigationand examination conducted by the inventors show the strain-aginghardenability significantly varies at an average grain size of theferrite phase in the ferrite and pearlite structure exceeding 15 μm evenwhen the amount of solute nitrogen is maintained at a predeterminedlevel. Moreover, degradation of the mechanical characteristics afterstorage at room temperature is significant. Although the details of themechanism are not known at the present, one of the causes of thevariation in the strain-aging hardenability is assumed to be related tothe grain size and is thus relevant to the segregation and precipitationof alloy elements in the grain boundaries and to the effects of theprocessing and heat treatment. In order to stabilize the strain-aginghardenability, the average grain size of the ferrite phase needs to benot more than 15 μm. In order to stably increase the bake-hardeningincrement and ΔTS, the average grain size is preferably not more than 12μm.

[0076] The cold-rolled steel sheet of the present invention having theabove-described composition and structure is excellent in thestrain-aging hardenability and exhibits a tensile strength TS of lessthan 440 MPa.

[0077] In regulating the strain-aging hardenability, the prestrain(predeformation) is the important factor. The inventors have studied theeffect of the prestrain on the strain-aging hardenability while takinginto account types of deformation employed in the steel sheets forautomobiles. The inventors have found that the effect of the prestraincan be classified in terms of uniaxially converted strain except for thecase of extensive deep drawing. The inventors have also found that inactual components, the uniaxially converted strain exceeds 5% and thatthe component strength displays good correspondence with the strengthobtained after the strain-aging at the prestrain of 5%. Thus, in thepresent invention, the predeformation in the strain-aging process isperformed at a tensile strain of 5%.

[0078] The standard condition of the conventional paint-baking processis a temperature of 170° C. held for 20 minutes. In the case where astrain of not less than 5% is applied to the steel sheet of the presentinvention containing a large amount of solute nitrogen, hardening isachieved even with the moderate (lower temperature) aging treatment. Inother words, the conditions of aging can be relaxed. Moreover,generally, heating at a higher temperature and holding the temperaturefor a longer period of time are effective in attaining higher hardeningas long as softening does not occur due to excessive aging.

[0079] To be more specific, in the invention steel sheet, the lowerlimit of the heating temperature which attains remarkable hardeningafter predeformation is approximately 100° C. No improvement inhardening is observed at a heating temperature exceeding 300° C. At atemperature exceeding 400° C., the steel sheet tends to soften, andoccurrence of distortion due to heat and temper color become noticeable.For the holding time, sufficient hardening is attained at a heatingtemperature of approximately 200° C. held for 30 seconds in many cases.For more stable hardening, the holding time is preferably not less than60 seconds. At a holding time exceeding 20 minutes, no furtherimprovement in hardening is observed, but the production efficiency issignificantly impaired as to be impracticable.

[0080] In view of the above, in the present invention, conventionalpaint-baking process conditions wherein the heating temperature of 170°C. is held for 20 minutes are employed as the aging condition forevaluation. The invention steel sheet stably attains a large degree ofhardening under aging conditions of low-temperature heating and shorterholding time where no conventional steel sheet for paint-baking attainsa sufficient degree of hardening. No limit is imposed as to the heatingmethod. Examples of the preferred heating method are heating in airusing a heater for standard paint-baking, induction heating,nonoxidation heating, laser heating, and plasma heating.

[0081] The automobile components need to have sufficient strength forresisting complex stress loads from the outside. Accordingly, both thestrength characteristics of the low strain zone and the strengthcharacteristics of the high strain zone are important in the materialsteel sheet. In view of the above, the inventors have determined thatthe invention steel sheet as the material for the automobile componentshas a BH increment of not less than 80 MPa and ΔTS of not less than 40MPa Preferably, the BH increment is not less than 100 MPa and ΔTS is notless than 50 MPa. In order to improve the BH increment and ΔTS, theheating temperature during aging may be increased, and/or the holdingtime may be longer.

[0082] The invention steel sheet before forming will not deteriorate byaging (a phenomenon in which YS is increased and El, i.e., elongation,is decreased) after being left to stand in room temperature forapproximately a year. This is a distinct advantage of the presentinvention not achieved in known art.

[0083] The effects of the present invention may be displayed in thesteel sheet having a relatively large product sheet thickness At aproduct sheet thickness exceeding 3.2 mm, however, an appropriatecooling rate cannot be maintained during cold-rolled sheet annealingprocess, strain aging occurs during continuous casting, and a targetstrain-aging hardenability as a product is barely attained. Thus, thethickness of the invention steel sheet is preferably not more than 3.2mm.

[0084] In the present invention, electroplating or hot-dip plating maybe performed on the surface of the invention cold-rolled steel sheetdescribed above. The resulting plated steel sheet will exhibit the samedegree of TS, BH increment, and ΔTS. Examples of suitable plating areelectrogalvanizing, hot-dip galvanizing, alloying hot-dip galvanizing,hot-dip aluminum plating, electric tin plating, electric chromiumplating, and electric nickel plating.

[0085] A method for manufacturing the invention steel sheet will now bedescribed.

[0086] The invention steel sheet is manufactured essentially bysequentially performing: a hot-rolling process of making a hot-rolledsheet, the process including rough-rolling a heated steel slab havingthe above-described composition to make a sheet bar, finish-rolling theresulting sheet bar, and coiling the sheet bar; a cold-rolling processof making a cold-rolled sheet, the process including pickling andcold-rolling the hot-rolled sheet; and a cold-rolled sheet annealingprocess including continuous casting and aging the cold-rolled sheet.

[0087] The slab employed in the invention manufacturing method ispreferably made by means of continuous casting so as to prevent macrosegregation of the components. Alternatively, ingot-making method orthin-slab continuous casting method may be used. The present inventionmay be applied to a standard process of cooling the prepared slab toroom temperature and reheating the slab, and to a energy-saving processsuch as direct rolling in which a hot slab is inserted to a heatingfurnace without cooling or is directly rolled after a brief period ofheat insulation. The direct rolling is especially useful to effectivelysecure solute nitrogen since precipitation of nitrogen is delayed inthis process.

[0088] First, restrictions on the hot-rolling process conditions areexplained.

[0089] Slab Heating Temperature: Not Less Than 1,000° C.

[0090] The slab heating temperature is preferably not less than 1,000°C. to secure a necessary and sufficient amount of solute nitrogen at theinitial stage and to satisfy the target amount of solute nitrogen, i.e.,not less than 0.0010%, in the final product. From the point of view ofpreventing an increase in loss caused by an increase in the oxidationweight, the slab heating temperature is preferably not more than 1,280°C. The slab heated under the above-described conditions is made into asheet bar by rough rolling. Rough rolling may be performed understandard known conditions, i.e., no special restrictions are imposed. Ashorter processing time is preferred to secure the required amount ofsolute nitrogen.

[0091] Next, the sheet bar is finish-rolled to prepare a hot-rolledsheet.

[0092] In the present invention, between rough rolling and finishrolling, a sheet bar is preferably joined to other sheet bars precedingand following that sheet bar to allow continuous finish rolling.Examples of the preferable joining method are pressure welding, laserwelding, and electron beam welding.

[0093] This will reduce the ratio occupied by nonstationary parts, i.e.,the front tip and the rear tip of the workpiece, which tend to displayirregularities in their shapes during finish rolling and the subsequentcooling, will increase the stable rolling length, i.e., the continuouslength up to which the same rolling conditions can be used, and thestable cooling length, i.e., the continuous length up to which coolingcan be performed under application of tension, and will improve yieldand the accuracy of shapes and dimensions. Moreover, lubricating rollingof a thin and wide strip, which has been difficult to perform due topoor rolling and biting characteristics according to a known rolling inwhich rolling is performed sheet by sheet, can be performed. Thus, therolling load and the roller surface pressure are reduced, therebyextending the life of the rollers.

[0094] In the present invention, preferably, either one or both of asheet bar edge heater for heating the ends of the sheet bar in the widthdirection and a sheet bar heater for heating the ends of the sheet barin the longitudinal direction are used between rough rolling and finishrolling in order to make the temperature distribution in the widthdirection and the longitudinal direction of the sheet bar uniform. Inthis manner, the variation in the material characteristics will befurther decreased. The sheet bar edge heater and the sheet bar heaterare preferably of an induction-heating type to achieve stable operation.

[0095] In operation, the temperature difference in the width directionis preferably corrected first using the sheet bar edge heater. At thistime, the heater is preferably adjusted such that the temperaturedistribution range in the width direction is not more than approximately20° C. at the exit of finish rolling, although the adjustment may differdepending on the composition of the steel. The temperature difference inthe longitudinal direction is then adjusted using the sheet bar heater.At this time, the heater is preferably adjusted such that thetemperature at the ends in the longitudinal direction is 20° C. to 40°C. higher than the temperature of the center portion.

[0096] Finish-Rolling Delivery Temperature: Not Less Than 800° C.

[0097] The temperature FDT at the delivery of the finish rolling is setat not less than 800° C. to prepare a steel sheet having a homogeneousand fine structure. At a finish-rolling delivery temperature below 800°C., the structure will become inhomogeneous due to generation of apearlite band, and a processing structure may remain in some portions.Remaining of the processing structure can be avoided by using highercoiling temperatures. However, at higher coiling temperatures, thegrains become coarse, the amount of solute nitrogen decreases, and themechanical characteristics display larger planar anisotropy. Thetemperature FDT is preferably not less than 820° C. to further improvethe mechanical characteristics.

[0098] Cooling After Finish Rolling: Quenching at a Cooling Rate of NotLess Than 30° C./s After Completion of Finish Rolling

[0099] Air cooling may be performed after finish rolling. Preferably,quenching at an average cooling rate of not less than 30° C./s isperformed after finish rolling. Under such conditions, the hightemperature range at which AlN precipitate can be quenched toeffectively secure nitrogen in the state of solid solution.

[0100] Coiling Temperature: Not More Than 650° C.

[0101] As the coiling temperature CT decreases, the strength of thesteel sheet increases, thereby allowing solute nitrogen to stably remainin the steel sheet. In order to securely improve the strain-aginghardenability, the coiling temperature is preferably not more than 650°C. At a coiling temperature of less than 200° C., the shape of thecoiled steel sheet becomes irregular, and the homogeneousness of thematerial characteristics is impaired, which is not an effect preferredin actual operations. Thus, the coiling temperature is preferably notless than 200° C. In the case where the requirement of thehomogeneousness of the material characteristics is tighter, the coilingtemperature is preferably not less than 300° C., and more preferably,not less than 400° C.

[0102] In this invention, during finish rolling, lubricating rolling maybe performed to reduce the hot-rolling load and to eventually stabilizethe strain-aging hardenability. The lubricating rolling has anadditional effect of making the shapes and the material characteristicsof the hot-rolled sheet more homogeneous. The coefficient of frictionduring lubricating rolling is preferably in the range of 0.25 to 0.10.The combination of the lubricating rolling and the continuous rollingwill lead to reliable hot-rolling operations.

[0103] The hot-rolled sheet subjected to the above-described hot rollingprocess is next subjected to a cold-rolling process in which thehot-rolled sheet is subjected to pickling and cold rolling.

[0104] Standard known conditions may be employed in pickling, and nospecial restrictions are imposed. In the case of cold-rolling asignificantly thin hot-rolled sheet, a cold-rolling process may bedirectly performed without pickling.

[0105] Standard known conditions may be employed as the cold-rollingconditions, and no special restrictions are imposed. To secure thehomogeneous structure, the cold reduction rate is preferably not lessthan 40%. The cold-rolled sheet is then subjected to a cold-rolled sheetannealing process including the steps of continuous annealing, coolingafter soaking, and optional overaging.

[0106] Continuous Annealing Temperature: Between the RecrystallizationTemperature and 950° C.

[0107] The annealing temperature during continuous annealing is set atnot less than the recrystallization temperature.

[0108] An annealing temperature below the recrystallization temperaturewill lead to incomplete recrystallization and low ductility, though thestrength is satisfactory. Accordingly, the steel sheet will exhibitdegraded formability and cannot be employed as the steel sheet forautomobiles. Preferably, the continuous annealing temperature is notless than 700° C. in order to further improve formability. On the otherhand, a continuous annealing temperature exceeding 950° C. will lead tosignificant irregularity in the shape of the steel sheet. In view of theabove, the continuous annealing temperature is preferably set betweenthe recrystallization temperature and 950° C.

[0109] Holding Time at the Continuous Annealing Temperature: 10 to 120Seconds

[0110] The holding time at the continuous annealing temperature ispreferably as short as possible in order to achieve a fine structure andthe desired amount or more of solute nitrogen. To secure stableoperation, the holding time is preferably not less than 10 seconds. At aholding time exceeding 120 seconds, a fine structure is barely obtainedand a sufficient amount of solute nitrogen is barely secured. In view ofthe above, the continuous annealing temperature is preferably held for10 to 120 seconds.

[0111] Cooling After Soaking: at a Cooling Rate of 10 to 300° C./s to aTemperature Zone of Not More Than 500° C.

[0112] In continuous annealing, cooling subsequent to soaking isessential for preparing a fine structure and securing a sufficientamount of solute nitrogen. In the present invention, during coolingafter soaking, a continuous cooling at a cooling rate of 10 to 300° C./sis performed to a temperature of not higher than 500° C. At a coolingrate lower than 10° C./s, a fine structure is barely obtained and adesired amount or more of solute nitrogen is barely obtained. On theother hand, at a cooling rate exceeding 300° C./s, a large amount ofsolute carbon remains, the yield strength YS increases, and theelongation El significantly decreases. Moreover, the materialcharacteristics become less homogeneous in the width direction of thesteel sheet. If cooling at a cooling rate of 10 to 300° C./s is stoppedat a temperature exceeding 500° C., a fine structure cannot be obtained.

[0113] Overaging may be performed after the cooling after the soaking.Overaging is not essential, but allows adjusting the amount of solutecarbon and the material characteristics such as yield strength andelongation related to the amount of solute carbon. Thus, overaging maybe performed if necessary for stabilizing material characteristics.

[0114] Overaging: Not Less Than 20 Seconds in the Temperature Zone of350° C. to 500° C.

[0115] By averaging, the amount of solute carbon can be reduced whilemaintaining the amount of solute nitrogen. Both solute nitrogen andsolute carbon are capable of yielding a remarkably high strain-aginghardenability, but room-temperature aging significantly occurs if theamount of solute carbons is large, resulting in degradation in thecharacteristics such as ductility and processability. In the presentinvention, solute nitrogen is primarily used to improve the strain-aginghardenability and to yield excellent mechanical characteristics. At anoveraging temperature below 350° C., the amount of solute carbon is notsufficiently decreased. At an overaging temperature exceeding 500° C., afine structure cannot be obtained. The effect of the overaging is notsufficient at an overaging time of less than 20 seconds. In view of theabove, the overaging is preferably performed in the temperature range of350° C. to 500° C. for not less than 20 seconds. The averaging time ispreferably not more than 600 seconds considering the line length of thecontinuous annealing furnace and other restrictions.

[0116] In the present invention, leveler processing or temper rollingboth at an elongation of 1.5% to 15% may be performed subsequent to thecold-rolled sheet annealing process. By performing temper rolling orleveler processing, free dislocations can be newly introduced, therebyreliably improving the strain-aging hardenability such as a bakehardening increment and tensile strength increment ΔTS. The totalelongation is preferably not less than 1.5% in the temper rolling orleveler processing. At an elongation below 1.5%, the yield strength ofthe steel sheet increases, thereby degrading ductility. The inventorshave confirmed that, although temper rolling is different from levelerprocessing, no significant difference is observed in the effectsregarding the strain-aging hardenability of the steel sheet.

[0117] The cold-rolled steel sheet of the present invention can beapplied to a plating steel sheet on which plating or alloying isperformed. The cycle of heating during alloying treatment corresponds tothe above-described averaging treatment. Thus, the resulting steel sheetdoes not deteriorate by room-temperature aging and exhibits a remarkablyimproved strain-aging hardenability.

EXAMPLES

[0118] A melt having the composition shown in Table 1 was prepared byusing a converter and was made into a slab by a continuous castingmethod. The resulting slab was heated under the conditions shown inTable 2, was subjected to rough-rolling to prepare a sheet bar havingthe thickness shown in Table 2, and was made into a hot-rolled sheet bya hot-rolling process in which finish rolling under the conditions shownin Table 2 was performed. Lubricating rolling was performed duringfinish rolling in some Examples. A sheet bar after rough rolling wasjoined to other sheet bars following and preceding that sheet bar at thedelivery of the finish rolling by means of fusion-pressure welding insome Examples. The temperature of the sheet bar is adjusted using asheet bar edge heater for heating the end portions in the widthdirection of the sheet bar and a sheet bar heater for heating the endportions in the longitudinal direction of the sheet bar, both heatersbeing of induction-heating type, in some Examples.

[0119] The resulting hot-rolled sheet was then subjected to pickling andto a cold-rolling process comprising cold rolling under the conditionsshown in Table 2 to prepare a cold-rolled sheet. The resultingcold-rolled sheet was then subjected to a continuous annealing using acontinuous annealing furnace under the conditions shown in Table 2. Thecold-rolled sheet annealing process was followed by temper rolling. Theannealing temperature in continuous annealing was not less than therecrystallization temperature in all cases.

[0120] The solute nitrogen content, the microstructure, the tensilecharacteristics, and the strain-aging hardenability in the resultingcold-rolled annealed sheet were examined.

[0121] (1) Examination of the Solute N Content

[0122] The content of solute N was determined by subtracting the amountof precipitated N from the total amount of N in the steel obtained bychemical analysis. The amount of precipitated N was determined by ananalysis employing the chronoamperometry described above.

[0123] (2) Microstructure

[0124] A sample piece was obtained from each cold-rolled annealed sheet.A microstructure image of the cross section of the sample piece taken inthe direction orthogonal to the rolling direction, i.e., Cross SectionC, is obtained using an optical microscope or a scanning electronmicroscope. The ratio and the type of the structure were examined usingan image analyzer.

[0125] In determining the grain size of ferrite, the grain sizecalculated from the structure image of the cross section taken in thedirection orthogonal to the rolling direction, i.e., Cross Section C, bya stereometry regulated by ASTM and the nominal grain size calculated bya sectioning method regulated by ASTM were determined, and the larger ofthe two sizes was employed as the grain size of ferrite.

[0126] (3) Tensile Characteristics

[0127] A Japanese Industrial Standard (JIS) No.5 sample piece was takenin the rolling direction from each cold-rolled annealed sheet and wassubjected to tensile testing according to JIS Z 2241 regulations at aninitial tensile rate of 3×10⁻³/s (cross-head rate: 10 mm/min, constant)to determine the yield strength YS, the tensile strength TS, and theelongation El.

[0128] (4) Strain-Aging Hardenability

[0129] A Japanese Industrial Standard (JIS) No.5 sample piece was takenin the rolling direction from each cold-rolled annealed sheet and wassubjected to predeformation at a tensile prestrain of 5%. Next, aheating treatment equivalent to a paint-baking process at a temperatureof 170° C. held for 20 minutes was performed. Subsequently, a tensiletesting was performed at an initial tensile rate of 3×10⁻³/s todetermine the tensile characteristics after predeformation andpaint-baking process, i.e., the yield stress YS_(BH) and the tensilestrength TS_(BH) and the BH increment=YS_(BH)−YS_(5%) and ΔTS=TS_(BH)−TSwere calculated. Here, YS_(5%) is the deformation stress afterpre-deforming the product sheet at 5% prestrain, and YS_(BH) and TS_(BH)are the yield stress and the tensile strength, respectively, after apredeformation-and-paint-baking treatment. The tensile strength of theproduct sheet is represented by TS.

[0130] The results are shown in Table 3.

[0131] All of the examples according to the present invention exhibitexcellent ductility and superior strain-aging hardenability and displaydistinctively high BH increments and ΔTS.

INDUSTRIAL APPLICABILITY

[0132] According to the present invention, a highly versatilecold-rolled steel sheet exhibiting high strain-aging hardenability andhigh formability in which the yield stress is increased by 80 MPa andthe tensile strength is increased by 40 MPa by predeformation andpaint-baking treatment can be manufactured at low cost withoutirregularities in the shape. Thus, the invention has a remarkableindustrial advantage. When the cold-rolled steel sheet of the presentinvention is applied to automobile components, both the yield stress andthe tensile strength can be increased by the paint-baking treatment orthe like, and stable characteristics as the components can be achieved.Another advantage is that the sheet thickness of the steel sheet usedcan be reduced to a thickness of 2.0 mm to 1.6 mm, for example, which isthinner than known sheets, thereby reducing the weight of automobilebodies. Moreover, the present invention has a remarkable industrialadvantage of facilitating hot rolling of thin sheets without increasingdeformation resistance since the strain-aging hardenability is improvedby adding nitrogen which does not significantly increase the deformationresistance at elevated temperatures. TABLE 1 Steel Chemical Composition(mass %) No. C Si Mn P S Al N N/Al Other Mn/Si Value A A 0.05 0.10 1.20.002 0.0015 0.010 0.0095 0.95 — 12 0.36 B 0.03 0.20 1.0 0.002 0.00200.015 0.0120 0.80 —  5 0.42 C 0.12 0.20 0.8 0.001 0.0015 0.009 0.00981.09 —  4 0.37 D 0.03 0.10 0.9 0.002 0.0015 0.009 0.0178 1.98 —  9 0.30E 0.05 0.02 1.5 0.001 0.0015 0.008 0.0095 1.19 Mo: 0.10 75 0.33 F 0.0050.07 1.2 0.002 0.0015 0.010 0.0120 1.20 Ca: 0.0035 17 0.33 G 0.015 0.101.2 0.003 0.0021 0.007 0.0079 1.13 Ti: 0.015 12 0.37 H 0.08 0.04 0.60.027 0.0028 0.012 0.0110 0.92 Nb: 0.009 15 0.43 I 0.05 0.10 1.1 0.0030.0012 0.012 0.0098 0.82 Ni: 0.07, REM: 0.0070 11 0.35 J 0.05 0.10 1.10.003 0.0022 0.021 0.0065 0.31 Cu: 0.5, Ni: 0.3 11 0.35 K 0.05 0.10 1.10.002 0.0021 0.031 0.0025 0.08 — 11 0.34 L 0.05 0.30 1.1 0.030 0.00150.010 0.0095 0.95 —   3.7 0.82

[0133] TABLE 2 Hot Rolling Process Average Rough Finish Rolling CoolingCoiling Slab Heating Rolling Delivery Hot-Rolled Rate Coiling SteelTemperature Sheet Bar Temperature Sheet After Temperature Sheet SteelSRT Thickness Sheet Bar FDT Thickness Rolling CT No. No. ° C. mm Welding° C. mm ° C./s ° C. 1 A 1200 38 Not Performed 840  2.6* 50 540 2 1180 38Performed** 840 2.6 35 540 3 1210 38 Performed 840 2.6 35 720 4 B 121035 Not Performed 855 2.6 40 550 5 1200 38 Not Performed 770 2.6 45 555 6C 1210 35 Performed 870 2.9 37 600 7 D 1200 35 Performed 865 2.9 38 6008 E 1170 35 Performed 850 3.2 35 600 9 F 1230 40 Not Performed 880 3.235 520 10  G 1220 40 Not Performed 890 2.6 35 520 11  1220 40 NotPerformed 880 2.6 35 520 12  H 1190 38 Not Performed 855 2.9 55 530 13 1190 38 Not Performed 858 2.9 50 530 14  I 1200 38 Not Performed 860 2.935 540 15  J 1200 38 Not Performed 860 2.9 35 540 16  K 1200 38 NotPerformed 865 2.9 35 540 17  L 1200 38 Not Performed 840 2.6 50 540Cooling Overaging After Retention Cold-Rolling Process ContinuousSoaking Time in Cold Cold-Rolled Anealing Cooling the Range Temper SteelReduction Sheet Annealing Holding Cooling Shut Down Starting of RollingSheet Steel Rate Thickness Temperature Time Rate Temperature Temperature350-500° C. Elongation No. No. % mm ° C. s ° C./s ° C. ° C. s %  1 A 710.75 780 60 20 480 480 30 0.5  2 71 0.75 800 50 20 470 470 40 0.5  3 710.75 880 50 20 490 490 45 0.5  4 B 73 0.70 800 40 15 450 450 40 0.5  573 0.70 800 40  3 450 450 39 0.5  6 C 76 0.70 720 40 20 450 450 50 1.5 7 D 76 0.70 720 40 25 450 450 50 1.0  8 E 78 0.70 720 40 25 450 450 291.5  9 F 75 0.80 750 50 20 450 450 30 1.5 10 G 69 0.80 750 40 20 450 45030 1.0 11 69 0.80 750 50 30 350 — — 1.0 12 H 72 0.80 730 50 20 440 44035 3.0 13 66 1.00 730 50 25 600 600 90 3.0 14 I 62 1.10 740 50 40 440440 40 0.5 15 J 62 1.10 750 50 40 440 440 40 0.5 16 K 62 1.10 820 50 40440 440 40 0.5 17 L 71 0.75 780 60 20 470 470 30 0.5

[0134] TABLE 3 Product Characteristics Steel Sheet After Solute N SheetStructure Characteristics Predeformation Strain Aging in FerritePearlite Tensile and Hardenability Steel Steel Area Grain AreaCharacteristics Paint-Baking BH Sheet Steel Sheet Ratio Size Ratio YS TSEl YR Value r YS TS Increment ΔTS No. No. MASS % % μm % MPa MPa % %r_(mean) MPa MPa MPa MPa Reference  1 A 0.0059 98 7 2 260 395 40 66 1.1410 455 95 60 IE  2 0.0050 98 7 2 254 390 41 65 1.1 415 452 95 62 IE  30.0007 97 17  3 255 385 34 66 1.2 355 410 55 25 CE  4 B 0.0055 99 8 1257 395 40 65 1.1 415 450 98 55 IE  5 0.0007 98 7 2 247 380 36 65 1.1345 400 45 20 CE  6 C 0.0090 96 6 4 285 425 39 67 1.0 450 482 90 57 IE 7 D 0.0146 96 6 4 287 435 38 66 1.0 475 516 120  81 IE  8 E 0.0079 98 72 281 420 40 67 1.1 445 479 102  59 IE  9 F 0.0065 >99  8 <1  231 355 4465 1.4 377 410 90 55 IE 10 0.0059 >99  7 <1  288 430 38 67 1.3 458 48990 59 IE 11 G 0.0065 >99  7 <1  290 438 38 66 1.3 455 497 85 59 IE 120.0055 98 5 2 270 395 41 68 1.0 421 457 100  62 IE 13 H 0.0008 98 5 3260 375 38 69 1.1 365 400 40 25 CE 14 I 0.0045 98 6 2 270 405 39 67 1.0421 455 90 50 IE 15 J 0.0048 98 6 2 275 415 39 66 1.0 425 460 80 45 IE16 K 0.0001 98 7 2 251 375 36 67 1.0 335 390 30 15 CE 17 L 0.0009 95 10 5 346 480 25 72 1.0 430 490 25 10 CE*

What is claimed is:
 1. A cold-rolled steel sheet having superiorstrain-aging hardenability and exhibiting a tensile strength of lessthan 440 MPa and a yield ratio YR of less than 70%, the steel sheetcomprising, by mass: not more than 0.15% carbon; not more than 0.4%silicon; not more than 2.0% manganese; not more than 0.04% phosphorous;not more than 0.02% sulfur; not more than 0.02% aluminum; and 0.0050% to0.025% nitrogen, wherein the silicon content, the manganese content, andthe phosphorous content satisfy relationship (1) below:Si+Mn/5+10P<0.44  (1) where Si, Mn, P represent contents, in terms ofpercent by mass, of corresponding elements, wherein the ratio N/Al isnot less than 0.3, the content of nitrogen in the state of solidsolution is not less than 0.0010%, the balance of the composition is Feand unavoidable impurities, and wherein the steel sheet has a structurecomprising the ferrite phase and the pearlite phase, the area ratiooccupied by the ferrite phase being not less than 90%, the average grainsize of the ferrite phase being not more than 15 μm.
 2. The cold-rolledsteel sheet according to claim 1, further comprising at least one groupselected from Group a to Group c below: Group a: at least one of Cu, Ni,Mo, and Cr, the total content being not more than 1.0%; Group b: atleast one of Nb, Ti, and V, the total content being not more than 0.1%;and Group c: at least one of Ca and REMs, the total content being0.0010% to 0.010%.
 3. A method for manufacturing a cold-rolled steelsheet having superior strain-aging hardenability and exhibiting atensile strength of less than 440 MPa and a yield ratio YR of less than70%, the method comprising: a hot-rolling process comprising: heating asteel slab to a temperature of not less than 1,000° C., the steel slabcomprising, by mass: not more than 0.15% carbon; not more than 0.4%silicon; not more than 2.0% manganese; not more than 0.04% phosphorous;not more than 0.02% sulfur; not more than 0.02% aluminum; and 0.0050% to0.025% nitrogen, wherein the Si content, the Mn content, and the Pcontent satisfy relationship (1): Si+Mn/5+10P<0.44 where Si, Mn, Prepresent contents, in terms of percent by mass, of correspondingelements, the ratio N/Al being not less than 0.3; rough-rolling theheated slab into a sheet bar; finish-rolling the sheet bar at afinish-rolling delivery temperature of not less than 800° C.; andcoiling the resulting sheet bar at a temperature of not more than 650°C. into a coiled hot-rolled sheet, a cold-rolling process comprisingpickling and cold-rolling the resulting hot-rolled sheet to prepare acold-rolled sheet, and a cold-rolled sheet annealing process comprising:annealing the resulting cold-rolled sheet at a temperature between therecrystallization temperature and 950° C. for a holding time of 10 to120 seconds; cooling at a cooling rate of 10 to 300° C./s to atemperature zone of not more than 500° C.; and optionally overaging inthe temperature zone of 350° C. to 500° C. for a retention time of notless than 20 seconds, wherein the hot-rolling process, the cold-rollingprocess, and the cold-rolled sheet annealing process are sequentiallyperformed.
 4. The method for manufacturing the cold-rolled steel sheetaccording to claim 3, wherein quenching at a cooling rate of not lessthan 30° C./s is performed before the coiling and after the finishrolling.
 5. The method for manufacturing the cold-rolled steel sheetaccording to claim 3 or 4, wherein temper rolling or leveler processingis performed at an elongation of 1.0% to 15% subsequent to thecold-rolled sheet annealing process.